Method and apparatus for improving the detectability of a broadcast control channel in a mobile communication system

ABSTRACT

A method for improving the delectability of a broadcast control channel of base in a radio system in which the transmitting and receiving frequency resources of respective transceiver units have been divided on the TDMA/FDMA principle into channels. The base stations have equipment for establishing radio connections to radio units in their respective radio coverage areas, and respective transmitters for transmitting a broadcast control channel. At least two of the base stations are arranged to transmit the broadcast control channel by utilizing the same predetermined frequency channel. In order to reduce interference, the transmission power used when transmitting on the predetermined frequency channel by the transmitters of the respective base stations is adjusted so as to be lower in those time slots in which the broadcast control channel is not transmitted, than it is in that or those time slots in which the broadcast control channel is transmitted.

This application claims benefit of international applicationPCT/FI95/00285 filed May 24, 1995

This application claims benefit of international applicationPCT/FI95/00285 filed May 24, 1995 published as WO95/33313 Dec. 7, 1995.

FIELD OF THE INVENTION

This invention relates to a method for improving the detectability of abroadcast control channel of a base station in a radio system in whichthe transmitting and receiving frequency resources of the transceiverunits a plurality of have been divided on the TDMA/FDMA principle intochannels, the base stations comprising means for establishing a radioconnection to radio units in their radio coverage area, and means fortransmitting a broadcast control channel, whereby the system includes atleast two base stations that have been arranged to transmit thebroadcast control channel by utilizing the same predetermined frequencychannel. The invention further relates to a cellular radio systemcomprising base stations, the transceiver units of which have theirtransmitting and resources divided on the TDMA/FDMA principle intochannels, the base stations comprising means for establishing a radioconnection to radio units in their radio coverage area, and means fortransmitting a broadcast control channel, whereby the system includes atleast two base stations that have been arranged to transmit thebroadcast control channel by utilizing the same predetermined frequencychannel.

BACKGROUND OF THE INVENTION

The invention is related to mobile communication systems divided intotraffic channels on the TDMA/FDMA principle (Time Division MultipleAccess/Frequency Division Multiple Access), and especially to the GSM(Groupe Special Mobile) system and DCS (Digital Cellular System). Theinvention is particularly related to a problem that arises when the samefrequency channels have to be used in several radio cells situated neareach other so that signals of different radio cells will intermingle anddisturb one another.

The BCCH channel (Broadcast Control Channel) of the GSM and DCS systemsis one example of a channel that is transmitted on the same frequencychannel by a base station of several radio cells situated near eachother. Base stations also use the frequency channel now being discussedfor other purposes, for example, as a normal traffic channel, since theBCCH channel needs only one available time slot. The base stationtransmits information on the broadcast control channel concerning therespective radio cell, such as information on the frequencies of trafficchannels to be used, for example.

In addition to the broadcast control channel of its own radio cell, aradio unit in a certain radio cell also listens to the broadcast controlchannel of the surrounding cells, decodes the received radio signals andperforms measurements concerning the power levels of the receivedsignals, that is, RSSI measurements (Received Signal StrengthIndication). For instance, handovers, i.e. transitions from one radiocell to another are performed on the basis of such measurements. In theGSM/DCS system, base stations transmit information only in the firsttime slot of the BCCH broadcast control channel, i.e. in time slot 0,whereby transceivers decode signals received only in that time slot.Other time slots are either used as traffic channels or fill frames aretransmitted in them. Bursts transmitted in each time slot of the BCCHchannel, i.e. even fill frames, are taken into account in the RSSImeasurements of the transceiver.

SUMMARY OF THE INVENTION

Since the base stations of radio cells situated near each other transmitthe BCCH channel by utilizing the same frequency channel, interferenceoccurs between signals transmitted by different base stations, whichwill particularly disturb the radio units situated farther away. Inpractice, it has been shown that because of the interference, radiounits situated farther have had problems with decoding faultlesslysignals on the BCCH channel transmitted in time slot 0.

The object of the present invention is to solve the above-mentionedproblem. This object is achieved with the method of the invention thatis characterized in that the transmission power of a predeterminedfrequency channel of the base station is adjusted lower in those timeslots in which the broadcast control channel is not transmitted than itis in that or those time slots in which the broadcast control channel istransmitted.

The invention also relates to a cellular radio system in which themethod of the invention can be utilized. The system of the invention ischaracterized in that at least one of the base stations comprisescontrol means for adjusting the transmission power of the aforesaidpredetermined frequency channel so as to be lower in that or those timeslots in which that predetermined frequency channel does not transmitthe broadcast control channel.

The invention is based on the idea that the mutual disturbance betweenbase stations situated near each other is significantly lowered if thetransmission power of that frequency channel which the base stationsutilize for transmitting the BCCH channel is lowered so that only thetime slots used for transmitting the BCCH channel are transmitted withfull power, and the other time slots of the same frequency channel aretransmitted with a lower transmission power. As the base stations of thesystem transmit only the essential time slots with full power and allthe other signals transmitted on the same frequency channel on a lowerpower level, disturbances and interference on that channel aresignificantly reduced on that channel, whereby even transceiverssituated farther away are able to decode faultlessly the informationtransmitted on the BCCH channel. The arrangement of the invention isespecially advantageous in such networks in which the time slots of abroadcast control channel are transmitted synchronized so that the cellssituated near each other, using the same broadcast control channel,transmit the time slots of the broadcast control channel at differenttimes.

If calls are also transmitted on the frequency channel of the broadcastcontrol channel, the quality of the calls may decrease slightly due to alower power level. This will not become a problem because in a situationof this kind the base station can perform a handover to such a trafficchannel on which the frequency channel of the broadcast control channelis not used at all.

The RSSI measurements of the transceiver units situated in theneighbouring cells naturally suffer somewhat from the fact thatdifferent time slots of the broadcast control channel are transmittedwith different power levels. This will not become a problem in mostcases, as the possible handovers and the corresponding procedures basedon the RSSI measurement are generally performed on the basis of theaverage of the RSSI measurements which naturally changes very little ina case in which just one or a few time slots have been transmitted on ahigher power level. Furthermore, the variations in the transmissionpower of the BCCH channel can be taken into account in handoveralgorithms. Possible unnecessary handovers, which may mainly occur whena transceiver unit is switched on after it has been switched off(whereupon it has to select the most suitable radio cell), will not haveany serious consequences as they will be corrected after a while by anew handover between the cells which will not cause any extra loading inthe network.

Hence, the most significant advantages of the method and the system ofthe invention are that the mutual disturbance of base stationsdiminishes, whereby radio units will be more reliable in decodinginformation transmitted on the broadcast control channel, and that atransceiver corresponding to the other transceivers (the sametransmission power) of the base station can be used for transmitting thebroadcast control channel, the lifetime of the transceiver will becomelonger and the consumption of energy lower due to use of a transmissionpower which is lower than the maximum power. When the transmission powerof the broadcast control channel is lowered, instead of it being raised,for example, in order to accomplish a corresponding situation, it is notnecessary to use a more powerful and more expensive transceiver thanother transceivers for transmitting the broadcast control channel.

BRIEF DESCRIPTION OF THE DRAWINGS

In the following, the invention will be described in more detail by wayof example by means of a first preferred embodiment of the systemaccording to the invention with reference to the accompanying figures,in which:

FIG. 1 illustrates the structure of a cellular radio system,

FIG. 2 is a detailed view of a part of the cellular radio system shownin FIG. 1, and

FIG. 3 illustrates the mutual synchronization of signals transmitted bythe base stations of FIG. 2.

DETAILED DESCRIPTION OF A PREFERRED EMBODIMENT OF THE INVENTION

FIG. 1 shows a diagram of a part of the GSM cellular radio system. TheGSM system is described in more detail for example in The GSM System forMobile Communications, by M. Moyle and M-B. Pautet, Palaiseau, France,1992, ISBN:2-9507190-0-7, for which reason it will not be described ingreater detail in this context.

FIG. 1 illustrates the re-use of frequency channels in the GSM system.In the case of FIG. 1, the frequency channels in use are divided so thatthe same frequency channels are used in each radio cell indicated by thesame letter (A, B or C) . That is, the same frequency channels are usedin all the cells indicated by the letter A. Therefore, the samefrequency channel is also used for transmitting a BCCH channel in theradio cells being discussed.

Circles A1, A2, A3 and A4 in FIG. 1 illustrate the area to which thedisturbance of the cells indicated by the letter A will extend when themaximum transmission power is used in them. Therefore, the radio unitsituated in the radio cell B, inside the circles A1, A2 and A3, hasproblems in known systems in connection with the reception and decodingof broadcast control channels of the neighbouring cells indicated by theletter A.

FIG. 2 is a detailed view of a part of the cellular radio system shownin FIG. 1. FIG. 2 shows only three base stations BTS1 to BTS3 of a radiocell indicated by the letter A in FIG. 1, and areas A1, A2 and A3 towhich their disturbance will extend when the base stations transmit withthe maximum power.

FIG. 2 shows that all the base stations BTS1, BTS2 and BTS3 make use ofthe same frequency channel F1 for transmitting the broadcast controlchannel, BCCH. A radio unit MS1 shown in FIG. 2 (situated in the radiocell B shown in FIG. 1) receives the BCCH channel transmitted by all thebase stations in FIG. 2. In a situation shown in FIG. 2, in addition tothe BCCN channel, the base station BTS1 transmits a call to a radio unitMS2 situated in the radio coverage area of the base station BTS1.

According to the invention, the transmission power of the broadcastcontrol frequency channel F1 of the base stations BTS1 to BTS3 shown inFIG. 2 is adjusted so that only the time slots of the broadcast controlchannel are transmitted with the maximum power. The other time slots ofthe BCCH frequency channel F1 are transmitted on a 1 to 5 dB lowerlevel.

FIG. 3 illustrates the mutual synchronization of signals transmitted bythe base stations BTS1, BTS2 and BTS3 of FIG. 2. FIG. 3 shows that thebase stations are mutually synchronized so that they transmit thebroadcast control channel, i.e. the BCCH channel at different times.Therefore, the base station BTS1 transmits the BCCH channel on frequencyF1 and in time slot 1. In accordance with the invention, that time slotis transmitted with the maximum power, that is, in practice with thesame transmission power as is used on the other frequency channels ofthe base station. In time slot 2 the base station BTS1 transmits a callto the radio unit MS2 shown in FIG. 2. In accordance with the invention,the base station BTS1 transmits time slot 2 (and time slots 3 to 6 ifthere is traffic in them) of frequency channel F1 on a lower power levelthan time slot 1. In the case of FIG. 2, time slots 3 to 8 are free,that is, the base station BTS1 does not transmit anything during them.

FIG. 3 further shows that the base station BTS2 transmits the BCCHchannel in time slot 2 and that the base station BTS3 transmits the BCCHchannel in time slot 3. The other time slots of frequency channel F1 ofthe base stations BTS2 and BTS3 are free but when they are taken intouse, the base stations BTS2 and BTS3 comprise a control means whichadjusts the transmission power in time slots 1 and 3 to 8 of the basestation BTS2 and in time slots 1 to 2 and 4 to 8 of the base stationBTS3 lower than in the time slot in which these base stations transmitthe BCCH channel. Therefore, the base stations BTS1 to BTS3 transmitonly the BCCH time slot with full transmission power. The radio unit MS1is thus capable of receiving and decoding the BCCH channel transmittedby the base station BTS2 in time slot 2 faultlessly even though the basestation BTS1 is at the same time transmitting a call to the radio unitMS2 on the same frequency channel F1. This is possible because the basestation BTS2 transmits, according to the invention, the BCCH channelwith the maximum transmission power, and the base station BTS1transmits, according to the invention, in time slot 2 with a loweredtransmission power. The base stations of FIG. 2 are thus mutuallysynchronized so that each of them transmits the BCCH channel in its ownturn and with full power. The base stations BTS1 to BTS3 do not evenmomentarily transmit simultaneously with the maximum power on frequencychannel F1. The adjustment of the transmission power can be accomplishedin the base stations BTS1 to BTS3 by utilizing a dynamic poweradjustment known per se, whereby a base station controller, for example,can act as a control unit controlling the transmission power of the basestations.

It is to be understood that the specification above and the figuresrelated thereto are only meant to illustrate the present invention. Thedifferent variations and modifications of the invention will be apparentfor those skilled in the art without deviating from the scope and spiritof the invention presented in the appended claims.

We claim:
 1. A method for improving the detectability of a broadcastcontrol channel of each of a plurality of a base stations in a radiosystem in which the transmitting and receiving frequency resources ofrespective transceiver units have been divided on the TDMA/FDMAprinciple into a plurality of channels, the base stations eachcomprising equipment for establishing a radio connection to radio unitsin respective radio coverage areas of the respective base stations, anda transmitter for transmitting a broadcast control channel, at least twoof the base stations being arranged to transmit the broadcast controlchannel by utilizing a same pre-predetermined frequency channelcomprising:adjusting the transmission power used by each respectivetransmitter for transmitting on said predetermined frequency channel byeach of said at least two base stations so as to be lower in those timeslots in which the broadcast control channel is not transmitted by thetransmitter of the respective base station than said transmission poweris in that or those time slots in which the broadcast control channel istransmitted by the transmitter of the respective base station.
 2. Themethod according to claim 1, wherein:transmissions by respective ones ofsaid at least two base stations in the time slots of said predeterminedfrequency channel in which the broadcast control channel is nottransmitted, are transmitted at a level which is 1 to 5 dB lower thanare respective transmissions in those time slots of said predeterminedfrequency channel in which the broadcast control channel is transmittedby the transmitter of the respective base station.
 3. The methodaccording to claim 1, wherein:each said base station is a base stationof Digital Communication System or Groupe Special Mobile mobilecommunication system.
 4. The method according to claim 1, furthercomprising:mutually synchronizing said at least two base stations sothat they transmit the broadcast control channel at different times. 5.A cellular radio system, comprising:a plurality of base stations, havingrespective transceiver units the transmitting and receiving frequencyresources of which are divided on the TDMA/FDMA principle into aplurality of channels, each of said base stations comprising means forestablishing a radio connection to radio units in a respective radiocoverage area, and means for transmitting a broadcast control channel,said base stations including at least two base stations arranged totransmit the broadcast control channel by utilizing a same predeterminedfrequency channel, at least one of said base stations comprising controlmeans for adjusting the transmission power of transmissions by therespective transmitter on said predetermined frequency channel so as tobe lower in that or those time slots in which the respective transmitterdoes not transmit the broadcast control channel.
 6. The cellular radiosystem according to claim 5, wherein:said at least two base stationseach include control means arranged to adjust the transmission power sothat the time slots of said predetermined frequency channel in which thebroadcast control channel is not transmitted, are transmitted on at alevel which is 1 to 5 dB lower than are transmissions by the respectivetransmitter in those time slots of the frequency channel in which thebroadcast control channel is transmitted.
 7. The cellular radio systemaccording to claim 5, wherein:said system is Groupe Special Mobile orDigital Cellular System cellular radio system.